Steel bars are supplied in “standard lengths” of 12 m, 15 m, or 18 m and in “standard bundle weights” of generally 2 ton bundles for the same size bars. In the case of concrete-reinforcing bars, they still have to be cut into shorter lengths according to the design of the building structure for a specific column, beam or floor slab. These short specific lengths are generally not designed to be multiples of any standard lengths and therefore will result in end losses when cut. Such cut-to-length operation is normally performed in a “Cut and Bend” yard, on or off the construction site. In a “Cut and Bend” operation, it is typical to expect a 5% loss in the steel bar ends, even with the best computer aided optimization programs. This 5% is very substantial in the building project, especially with the current high prices of steel bars.
The standard lengths of 12 m, 15 m and 18 m are selected to make maximum use of the size of the cargo holds of ships and trucks, while the standard bundle weight also serves for optimizing the capacity of the lifting apparatus of the ships and trucks.
There is an existing technology salt called “Flash Welding” of hot billets in which the tail end of a billet is welded to the front end of the next billet, in-line as they exit from the reheating furnace. Such operation will enable the rolling of bars continuously without any end; this is similar to “sequence casting” in a continuous casting machine. The primary objective of this process of endless rolling is to minimize the head and tail crops in the rolling mill and the short end losses at the cooling bed. In employing this flash welding process together with the very high accuracy of the modern flying shear before the cooling bed, one is able to achieve an accuracy of +50/−00 mm consistently for 120 m long bars on the cooling bed. This is about 0.05% end losses versus an industrial norm of 2.5%.
The flash welding process produces elevated temperature at each of the flash welded joints of the billet. Due to the temperature dependence of draft/spread characteristics, the joints, being at a higher temperature than the rest of the billet, will have more elongation than spread, resulting in “necking”—smaller cross sectional area than the nominal area in the finished product. This means that the area of the joint can fall below the minimum area specified by the Steel Standard. U.S. Pat. No. 6,929,167 B2 Pong et al. teaches a method which will eliminate such effect of such necking. In the actual operation, gauges were set up in the rolling line to monitor the dimension of the steel bars and the results show that this method is able to maintain a uniform cross section throughout the length of the bar including the flash welded joints.
With the use of flash-welding and uniform cross-section at the weld joints, “In-Line Cut to Length” steel bars are produced by the rolling mills with no loss of production and with substantial reduction in material losses.
Steel bars are generally rolled from a 150 mm (6 inches) square billet of 12 meter (40 feet) length. These billets will be rolled into finished steel bars of various diameters from 50 mm (2 inches) to 10 mm (⅜ inch). Because of the starting weight of the billet is finite, it will end up with various finished lengths of the steel bars of each diameter. The total lengths of the steel bars from each piece of billet will not be exact multiples of the normal finished bar length of 12 m (40 feet) and one will have end losses. This is usually 2.5%. With endless rolling as described above, the billet is welded end to end to form a continuous infinite piece and there will be no end losses.
In the specifications of steel bar standards BS4449 or ASTM 615, a size tolerance is permitted, provided it does not compromise the strength of the steel bar. Typically, BS4449 (2005) such allowable size tolerance is plus or minus 4.5%. The aim is to target at minus 3%, i.e. a lighter bar, but without affecting the strength of the bar at the nominal diameter. With single billet rolling, the minus 3% of the finished bar diameter will end up with a longer end piece by the same 3%. This will go to waste. With the endless rolling of welding the billets, the longer end piece will go into and become part of the next incoming piece and therefore this minus 3% is completely recovered as usable steel and is not waste.
For shipping or trucking reasons, such finished steel bars are usually cut to 12 m (40 feet), 15 m (50 feet), or 18 m (60 feet) lengths. Because of different lengths of columns, beams or slabs, the actual length requirement at the building site is never exactly 12 m, 15 m or 18 m. These bars have to be cut in a separate operation. A typical end loss of cutting to specific bar lengths is 5%.